1. Field of the Invention
The present invention relates to an automotive discharge bulb having a light emitting tube that includes a ceramic tube in which electrodes are oppositely placed. The light emitting tube is filled with a light emitting material and a starting rare gas. The present invention further relates to an automotive headlamp having the discharge bulb.
2. Description of the Related Art
Generally, a discharge bulb having a glass arc tube is used as a light source for an automotive headlamp. However, the related art discharge bulb has encountered problems. For example, but not by way of limitation, metal halide filled in the light emitting tube promotes corrosion of the light emitting tube (or glass tube). Further, blackening and devitrification phenomena occur. Thus, no appropriate light distribution is obtained, and the life of the discharge bulb is not very long.
Recently, there has been proposed a related art discharge bulb (see FIG. 18) having a light emitting tube 110 that includes a right-circular-cylindrical ceramic tube 120, of which both ends are sealed with cylindrical insulating elements 130, and in which electrodes 140, 140 are oppositely placed, and that is filled with a light emitting material together with a starting rare gas, as described in JP-A-2001-76677 (see Paragraph No. 0005 of the specification and FIG. 5 thereof). The ceramic tube 120 is stable against metal halide, and has a long life, as compared with the glass arc tube.
Naturally, the discharge bulb for use in an automotive headlamp is required to have a good rising characteristic of luminous flux to obtain predetermined luminous flux immediately after the lamp is turned on. This is the same with the discharge bulb having a light emitting tube constituted by a right-circular-cylindrical ceramic tube, which is described in JP '677. This discharge bulb is required to have the ceramic tube, whose diameter is relatively small (whose enclosed space has small capacity), so as to improve the rising characteristic of luminous flux.
However, an arc generated by discharge between the electrodes has an upwardly convex shape. Therefore, as the diameter of the ceramic tube is decreased, the central part of high temperature arc is brought into substantial contact with a tube wall. Thus, the ceramic tube is required to have higher thermal shock resistance. Consequently, very limited ceramic materials are available as the material of the ceramic tube of the light emitting tube.
The related discharge bulb has another problem that when the central part of the high-temperature arc is put into large contact with the tube wall, an amount of heat radiated from the tube wall increases, and this increase in the amount of radiated heat delays the rise in luminous flux. Thus, the rising characteristic of luminous flux is degraded.